The field of this invention pertains to preparation of and materials for electrical contacts. Specifically, electrical contact materials comprising silver combined with small amounts of CeO.sub.2 is the subject matter of this invention.
Pure silver, or high content silver alloys, such as silver/cadmium oxide, are extensively used for switch and relay applications. Any material which is a candidate for use as make and break contacts must have characteristics of low wear erosion and low tendency to stick or weld at fairly low current values. To meet the requirements of specific applications, such as sensitive relays, switches, thermostats, motor starters, contactors, circuit breakers and other electrical devices, many silver contact materials have been used, such as silver alloyed with other metals, or having metals or oxides or graphite as a dispersed phase therein.
Commercial silver alloys developed for specific applications are based on the best compromise of contact erosion, welding tendency, and low contact resistance (heat rise) and are generally limited to a maximum current value of 10-15 amperes when long life (10.sup.6 operations) is required. Failure or end-of-life in these applications is generally reached because of (1) formation of a spike and crater erosion pattern, which may lead to bridging the contact gap and result in an interlocking type of weld; (2) actual welding of the contacts, which is considerably enhanced by excessive erosion, or the formation of small molten globules or whiskers on the contact surface and edges; or (3) the development of high and variable contact resistance which results in excessive heat or temperature rise.
It is known that the elevated temperature strength and hardness of metals can be significantly increased by the addition of a finely dispersed stable oxide phase. Theories of dispersion strengthening are well developed and good agreement of experimental data with theory has been observed. However, the effect of these oxides on the important electrical contact characteristics, such as arc erosion, weld tendency, and change in contact resistance, is little known and less understood. Silver-cadmium oxide is a contact material of this type, consisting of CdO dispersed in a silver matrix. Hovever, silver-cadmium oxide is in a special category, since CdO is not a stable oxide such as is required for dispersion strengthening, particularly at elevated temperature. In silver-cadmium oxide contacts the CdO phase is volatile and decomposes (at approximately 1700.degree.-1750.degree. F) during arcing; this feature gives this material its unique arc-quenching characteristics, especially when used in heavy current applications of 10-50 amps and higher. It should also be noted that these materials contain a fairly high oxide content, usually 10-15%. Even when present in small amounts there is no appreciable strengthening effect of CdO on silver, and above 15% CdO these alloys are too brittle to be fabricated by conventional methods. One of the outstanding properties contributed by CdO to silver is that it decreases the amount of material lost by arc erosion.
The prior art has been relatively silent in using CeO.sub.2 as an agent for strengthening silver to yield an electrical contact material. U.S. Pat. No. 2,545,438 issued to M. J. Stumbock et al. discloses oxides of cerium in combination with silver for use as a spark plug electrode. That use is different from the subject matter of this disclosure in that a mechanical electrical contact has two contacts physically opening and closing with resulting electrical arcs.
A Japanese Patent Application of I. Morimoto et al., bearing Application No. Sho 47/1972-26542, dated Mar. 17, 1972, and Open Patent Publication No. Sho 48/1973-95311, dated Dec. 7, 1973, discloses an electrical contact material comprising silver and oxides of La, Ce, Pr, Nd, and Sm. Specifically the Japanese application shows electrical contact materials comprising silver and up to five mol percent of the oxides of La and Sm, having atomic numbers of 57 and 62 respectively, and since Ce has an atomic number of 56, the applicants assumed that CeO.sub.2 would have a similar effect. The Japanese application teaches the preparation of silver-cerium group oxide compositions according to an internal oxidation process. This process begins by melting silver and one or more pure cerium group metals, and subsequently heating the alloy in air in a Tammann oven at 650.degree. C for 250 hours, thereby selectively internally oxidizing the cerium group metal.
The composition which results from the process described above, e.g. Ag/CeO.sub.2, was described in the Japanese application as having comparable arc-erosion, contact resistance and anti-welding properties with the well-known electrical contact material, silver and 12 weight percent cadmium oxide. The Japanese Open Patent Publication mentioned above describes the serious problem of preparing silver/cerium group oxide alloys by the internal oxidation process, especially where percentages of cerium group metals reach five atomic percent or more with silver. A crack in the contact may be caused by volume expansion due to the internal oxidation.
It is a general object of this invention to provide an improved powder metallurgy process for making an Ag/CeO.sub.2 electrical contact material from which to fabricate an electrical contact having high resistance to arc erosion and welding with acceptable electrical contact resistance.
It is a specific object of this invention to provide an electrical contact made from a material prepared by a coprecipitating, admixing or coating step to yield a powder mixture of Ag and CeO.sub.2 in desired proportions and a consolidating step to transform the powder to wire or sheet electrical contacts having parameters of low arc erosion and welding and low and stable surface contact resistance.